Method for manufacturing laminated hd (high-density) paper with good oxygen-barrier properties, and hd paper obtained thereby

ABSTRACT

Polyethylene applied to high-density paper where the surface of the paper is modified in order to have a surface energy as close as possible to polyethylene and thereby achieve properties as barrier material for packaging of food. The good barrier property is a result of crystallisation of polyethylene on the paper&#39;s surface.

This invention relates to a barrier material for use as packaging,particularly for articles of food, together with a method formanufacturing such a barrier material with good oxygen-barrierproperties.

BACKGROUND

Approximately 4/5 of all foodstuffs are susceptible to loss and/orabsorption of gasses, water vapour, flavour, odour, etc. and have to bestored in a protected environment.

Exposure to oxygen gas in particular can represent a serious problem,since oxygen is known to have a harmful effect on the flavour, texture,colour, nutritional content, etc. of foodstuffs. In addition, oxygen gasis instrumental in a number of known reactions which affect the storagedurability of food, such as microbial growth, colour changes, oxidationof lipids (rancidity) and rotting of fruit, etc. Foodstuffs of this kindtherefore have to be protected against the oxygen content of theatmosphere during storage, which in practice often means that thestorage time of the article of food is determined by the rate oftransmission of oxygen (often described as OTR) through the packaging.

Foodstuffs are often sold in small quantities in individually packagedunits, with the result that the cost associated with the packing andwrapping materials in itself becomes a very important competitivefactor. The importance of finding inexpensive packaging materials withan optimal balance between price and oxygen-barrier properties thereforebecomes a vital competitive factor for food producers. This has resultedin a steadily increasing demand for new improved packaging materialswhich are both inexpensive and have satisfactory barrier properties,with the result that rapid strides are being made in developments inthis field at the present time. Materials that have an OTR of 1 ml O₂/m²24 h bar or less are currently considered to be excellent for mostfoodstuffs.

In addition to the economic and functional aspects of food packaging,the environmental aspect will become more and more important since thefood industry has a very large sales volume with extensive use ofindividual packaging in small quantities. The sale of foodstuffs therebyresults in a far from insignificant amount of packaging waste which hastraditionally been dumped on rubbish tips. However, this solution is nolonger satisfactory since there are considerable problems associatedwith, for example, run-off to ground water from rubbish dumps, inaddition to which the increasing world population and resultant amountsof rubbish means that in many densely populated areas in the world thereis quite simply a lack of suitable dumps. More stringent demands aretherefore placed on packaging producers for their products to be made ofreproducible materials and to be recyclable or capable of beingincinerated safely and converted to heat energy.

PRIOR ART

A laminate of aluminium foil and, for example, cardboard is an exampleof a well-known and long-used packaging material with excellentcharacteristics (OTR values down to 0.003 ml O₂/m² 24 h bar, extremelyflavour-tight, etc.), which has problems with the new demands withregard to environmental standard. This is due, amongst other things, tothe fact that it is difficult to recycle the packaging material (thealuminium foil is too thin to be remelted) and it will produce toxicaluminium oxide when incinerated.

It has long been known that a laminate of polyolefines on some types ofpaper or cardboard can provide excellent barrier properties. Examples ofsuch solutions can be found in NO 166359 and EP 0754719 where paper ofgreaseproof quality is coated with polyethylene. This is a verypromising material for packaging food since it has the desiredcombination of favourable price, a high degree of reproducible rawmaterials, can be safely incinerated with a good caloric value withoutproducing toxic compounds and is easy to recycle.

The oxygen barrier for such materials will normally be just below 100 mlO₂/m² 24 h atm. This is rather striking since with a purely additivesummation of OTR for the paper and the polyethylene, which is the rulethat normally applies for laminates, an oxygen barrier B in the range1000-4000 ml O₂/m² 24 h atm is to be expected. In other words, some kindof favourable effect must be generated by coating the paper withpolyethylene.

In Norwegian patent application no. 19984666 (subsequently PCTapplication WO 00/20212) an explanation is suggested for this favourablebarrier effect that occurs when polyethylene is applied to paper with ahigh fibre content. It was suggested therein that the favourable barrierproperty is due to the formation of a crystalline layer in thepolyethylene. This was assumed to be a morphological change, whichoccurs due to sheer forces during the coating process and which arrangesthe polyethylene molecules at the surface of the paper, thus forming acrystalline layer in the polyethylene at the surface of the paper. Thislayer is also called a transcrystalline layer. On this basis patentclaims were formulated for a manufacturing process for such barriermaterials where the emphasis was placed on coating temperatures andsheer stresses. It was demonstrated that an OTR of around 25 ml O₂/m² 24h atm is obtained by means of this method, and sometimes as low as 1 mlO₂/m² 24 h atm, which is a substantial improvement over the processesdescribed in NO 166359 and EP 0754719. This explanation model wasprovided by the same inventor as for this invention. In the rest of theworld the prevailing theory for the formation of the oxygen barrier isthat the polyethylene seals the pores of the paper. This model will beclarified further in the following description of this invention.

Regardless of why this favourable barrier property is created when paperis coated with polyolefines, it is a fact that known methods forproducing such barrier materials are encumbered with a very greatvariance in OTR value when attempts are made to produce barriermaterials with the best OTR values. This indicates that we still do notknow how and why the barrier is created, thereby allowing chance to playa part in the production of barrier materials based on a laminate ofpolyolefine and paper/cardboard. With the knowledge that the storagedurability of oxygen-sensitive foodstuffs is directly proportional tothe packaging's OTR value, this uncertainty regarding the packaging'sOTR value becomes completely unacceptable for many food producers.

THE OBJECT OF THE INVENTION

It is therefore an object of this invention to provide a method forproduction of packaging materials comprising a laminate of a polyolefineon paper/cardboard where the formation of the oxygen-tight layer iscontrolled so as to provide consistent product qualities.

It is also an object of this invention to provide a packaging materialcomprising a laminate of paper and polyethylene which has very low aswell as very little variance in the oxygen transmission rate from oneproduction run to the next.

LIST OF FIGURES

FIG. 1 illustrates experimentally verified correlations between averagesurface energy of the paper and the polyethylene in a laminateconsisting of paper coated with polyethylene of type HDPE and the oxygentransmission rate (OTR) achieved for the laminate. The bottom curveapplies to a number of types of paper produced from wet pulp, while theupper curve illustrates the same for paper types produced from dry pulp.

FIG. 2 illustrates experimentally verified correlations between averagesurface energy of the paper and the polyethylene in a laminateconsisting of paper coated with polyethylene of type LDPE and the oxygentransmission rate (OTR) achieved for the laminate.

FIG. 3 depicts SEM photographs (200×) of the internal interface of apolyethylene film of type HDPE after the paper has been removed. Sample1 is a laminate that had an OTR value of 51.1 ml O₂/m² 24 h atm, whilesample 2 had an OTR value of 3.1 ml O₂/m² 24 h atm. Both samplesdemonstrate approximately the same degree of blistering.

FIG. 4 illustrates the correlation between the magnitude of themonoclinic fraction and the OTR value in the samples with the respectivepaper types of “wet” sulphite paper with different surface energy. Thepercentage of monoclinic crystallinity is increased with increased sheerforces, i.e. increased pressure on the squeeze roller. Candor: non-AKDhydrophobic. FL812/S.B.: hydrophobic, steaming chamber-treated paper.

DESCRIPTION OF THE INVENTION

The object of the invention can be achieved by means of a method asdefined in the patent claims and in the following description of theinvention.

The inventive method is based on research results carried out by theinventor in a doctorate project demonstrating that the barrier iscreated on account of morphological changes in polyethylene when thepolymer is applied to the paper. When the polymer mass is attached tothe surface of the paper before being subjected to rolling with pressureload, sheer forces are created that orientate the polymer molecules atthe surface of the paper. Together with cooling, this initiates theformation of a transcrystalline layer at the surface of the paper. Thusthe plastic cannot be regarded as a passive sealing material as in theprevailing explanation of the synergy effect between the plastic and thepaper. This feature is known from the inventor's earlier Norwegianpatent application no. 19984666 (subsequently PCT application WO00/20212).

The results, however, also illustrate two new features, one of which isthat the formation of the transcrystalline layer is promoted by the useof paper made of “wet” pulp, i.e. pulp that was not permitted to drybefore the production of the paper. In conventional paper production useis made almost exclusively of pulp with dried fibres in order, amongstother things, to reduce transport costs. However, the drying results ina hornification process where the fibres in the pulp spread out andbecome more crystalline and needle-shaped in long thin fibres. Theinventor's experiments demonstrate that this process is inopportune withregard to the formation of the gas-tight crystalline layer in thepolyolefine, and the use of dry fibres in the paper should therefore beavoided if a low OTR is required. This “hornification process” occurswhen the percentage of water in the pulp is less than approximately 30%by weight. Ready-dried pulp contains approximately 10% water by weight.Similarly, the term wet pulp is used for pulp that has from 30% water byweight or more, with an upper limit of pure water.

The second new feature is that it has been found that the differencebetween the paper's and the polyolefine's surface energy also has astrong influence on the formation of the gas-tight transcrystallinelayer. Here there is almost a linear correlation indicating that thesmaller the difference in surface energy, the denser the crystallinelayer. This effect is illustrated in the bottom curve in FIG. 1illustrating OTR value as a function of the average of the surfaceenergy of the paper and the polyolefine film for a number of types ofpaper coated with polyethylene of type HDPE. In the bottom curve onlypaper produced from wet pulp is plotted. Note that the curve approachesan OTR value equal to 0 for paper that has the same surface energy as aHDPE film, which is measured at 38.4 mN/m. Exactly the same tendency canbe seen in the curve in FIG. 2 illustrating the same but now fordifferent paper coated with polyethylene of type LDPE. Here too the OTRvalue approaches 0 for paper with the same surface energy as the LDPEfilm (36.2 mN/m). In the figures the curve is given as the average valueof the surface tension of both the paper and the polyethylene film, butthis curve has the same tendency and zero point as a curve based on onlythe paper's surface tension since the polyethylene's surface tensionremains the same regardless of which paper it is laminated to. The onlydifference is that the curve has a flatter profile, i.e. a smallergradient, which is irrelevant in this connection.

These are highly unexpected and striking results that conflict with theprevailing theory in the field. A person skilled in this field is notfamiliar with the significance of the method of manufacturing the paper,and has traditionally placed all the focus on the importance of theactual type of paper (which is of a standard type produced from drypulp) and how to treat it before lamination with the polyethylene. Thiscan be seen, for example, in two newly published technical articles (1and 2), where paper of the standard type (dry pulp) has been employedand the effect has been studied of the paper's surface energy on the OTRof the finished laminate. The importance of increasing the paper'ssurface energy is discussed there, since the results from articles 1 and2 show that better adhesion is obtained between the materials andimproved barrier properties the higher the surface energy of the paperis during the coating process. A person skilled in the art is aware thatthe paper's surface energy can be increased by various oxidationprocesses including treatment with ozone. This prevailing theory can beillustrated, for example, by the top curve in FIG. 1.

According to this prevailing theory the barrier properties are due tothe fact that the polyolefine seals the pores of the paper, and thegreater the surface energy of the paper surface, the more tightly thepolyethylene will seal the pores through improved adhesion. This theoryassumes that there is good contact between the materials in theinterface in order to achieve a minimum net area for the polyolefinefilm that is subjected to the passage of oxygen (a 25 μm polyethylenefilm has an OTR in the order of 7000 ml O₂/m² 24 h atm). This theory hasbeen tested by the inventor by studying the surface of coatedpolyethylene films and finds a considerable element of gas bubbles inthe polyethylene phase against the paper of around 6-8% of the totalarea. In addition samples have been found that have a good oxygenbarrier and a bubble-covered area that is at least as large as sampleswith a poor oxygen barrier. This is illustrated in FIG. 3 where sample 1has an OTR of 51.1 ml O₂/m² 24 h atm and sample 2 an OTR of 3.1 O₂/m² 24h atm. From the figures it can be seen that both samples manifestintensive blistering to approximately the same extent despite the greatdifference in OTR value. This is a clear indication that the prevailingtheory cannot be correct. The barrier effect must be due either tofactors radically different to pore sealing, or additional factors mustcome into the picture.

As mentioned, the inventor's studies of the polyolefine film aftercoating have shown that the barrier property is directly linked to theformation of the crystalline layer in the polyolefine at the surface ofthe paper. In a more detailed study it was found that this layer has amonoclinic crystal structure as opposed to the polyolefine's bulkvolume, which has orthorhombic unit structures. Furthermore, it wasfound that the percentage of monoclinic crystal structure in relation tothe percentage of orthorhombic unit structures also has an effect on thebarrier properties. The correlation is that the higher the percentage ofmonoclinic crystal structure, the lower the OTR value for the laminate.This is illustrated for a number of types of paper in FIG. 4, whichillustrates the OTR value for the laminate as a function of thepercentage of monoclinic crystal structure in the polyethylene film.From the inventor's earlier patent application no. NO 19984666(subsequently PCT application WO 00/20212), it is known that substantialsheer forces during coating promote the formation of monoclinic crystalstructure. A simple way of achieving substantial sheer stresses is toemploy strong roll pressure when applying the polyolefine film. The useis therefore preferred of a pressure of more than 250 kPa, and mostpreferred a roll pressure of 400 kPa or more, but it is also possible toachieve a good, dense crystalline layer by leaving it to time. It hasalso been shown that storage can give a substantial improvement in theOTR value. This can be seen from Tables 2 and 3 which can be found inExample 3 later in the description. These illustrate that storage lowersthe OTR value to the same value as that provided by a roll pressure of400 kPa. This may indicate that when the paper has the same surfaceenergy as the polyolefine film, the thermodynamic energy conditions inthe laminate will be favourable for the formation of the crystallinelayer (ΔG for the formation is less than 0). This is a great advantagesince when the layer is thermodynamically stable, the barrier propertieswill also become stable with time. The film will not lose its propertiesduring storage, but on the contrary the properties can be expected toimprove.

In conclusion, the inventive method is based on the recognition of thediscoveries that the barrier properties of laminates of the type wherepaper is coated with a polyolefine are attributable to the formation ofa transcrystalline layer in the polyolefine film at the surface of thepaper instead of pore-sealing in the surface of the paper, which is theprevailing theory. This layer, moreover, is promoted by a combination ofthe use of wet pulp during the production of the paper and theadjustment of the paper's surface energy during coating to approximateas closely as possible the polyolefine's surface energy. This usuallymeans reducing the paper's surface energy, which is completely contraryto general expert knowledge in the field that states that the higher thepaper's surface energy during coating, the lower the OTR values of thelaminate.

All known methods of surface treating paper in order to lower thesurface energy to the polyolefine's surface energy may be employed. Suchprocesses must be considered to come within the scope of the competenceof a person skilled in the art, and thereby within the concept of theinvention providing they are combined with the use of paper produced bymeans of wet pulp. These methods comprise steam treatment before glazing(calendring), internal sizing with a neutral hydrophobic agent, a highcomminution rate for the pulp, surface sizing or other chemical surfacetreatment of the paper.

The closest to optimal conditions for obtaining low OTR values forlaminate consisting of paper coated with polyethylene are created bymeans of highly comminuted sulphite cellulose pulp which is notpre-dried during the production of the paper, internal sizing of thepaper with a neutral hydrophobic agent and steam treatment of thesurface of the paper before the final calendring (rolling).

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in greater detail with reference toexamples of the method.

All numerical values of surface energies used in this application arecalculated on the basis of the development of contact angles in waterand tetrabrommethane on the surface of the respective material samples.These were measured with a Fibro DAT 1100 Mk II instrument according toTappi T558, ASTM D5725 at the paper industry's research institute inTrondheim, Norway. All measurements of oxygen transmission rates (OTR)were carried out according to ASTM D3985 (Standard test method foroxygen gas transmission) by means of the instrument OXTRAN 10/50. Themeasurements were conducted in the premises of Matforsk at Ås, Norway.

EXAMPLE 1 Oxygen Barrier in Laminate with Paper Made of Wet SulphitePulp Contra Dry Sulphate Pulp

Table 1 illustrates that paper made of wet sulphite pulp gives asignificantly better oxygen barrier than paper made of “dry” sulphatepulp with otherwise identical coating conditions. TABLE 1 Results ofmeasurements of OTR and HDPE-coated HD paper made of dry (sulphate) and“wet” (sulphite) pulp respectively. The surface energy for eachindividual paper is also given. A film of HDPE (CG8410) has a surfaceenergy of 38.4 mN/m. Surface energy Sample Paper OTR* (mN/m) a SuperPerga Parchment, 51.1 45.7 4440: “dry” sulphate, C b Super PergaParchment, 28.2 44.7 4450: “dry” sulphate, D c Super Perga Parchment,48.5 30.9 4460: “dry” sulphate, Z d Super Perga Parchment, 3.1 43.24470: “wet” sulphite, Z*Given as ml O₂/m² 24 h atm

EXAMPLE 2 Oxygen Barrier in Laminate with Paper Made of Wet SulphitePulp which is Treated in a Steaming Chamber Contra Laminate of SimilarPaper which is not Treated in a Steaming Chamber

Treatment in a steaming chamber lowers the surface energy of the paperand gives a slightly better barrier for HDPE. There is every indicationthat the contact between the materials is improved. This results in alowering of the average surface energy, thus lowering ΔG (Gibbs freeenergy) for the formation of new crystalline structures on the surfaceof the paper by means of storage (particularly for LDPE). This resultsin lower OTR. This can be seen in Tables 2 and 3. Here CG 8410polyethylene is of type HDPE and CA7230 is of type LDPE. The sameapplies in Table 4. TABLE 2 Results of measurements of OTR of PE-coatedHD paper made of “wet” (sulphite) pulp (FL812). CA7230: 36.2 mN/m,FL812: 40.4 mN/m Roll OTR OTR, Stored Polymer pressure kPa ml/m² 24 hatm ml/m² 24 h atm CG 8410 250 1.32 1.61 CG 8410 400 1.29 1.31 CA 7230250 3.49 2.02 CA 7230 400 1.59 1.41

TABLE 3 Results of measurements of OTR of PE-coated steamingchamber-treated HD paper made of “wet” (sulphite) pulp (FL812) 39.6 mN/mRoll OTR OTR, Stored Polymer pressure kPa ml/m² 24 h atm ml/m² 24 h atmCG 8410 250 1.55 1.55 CG 8410 400 1.07 0.92 CA 7230 250 8.13 2.36 CA7230 400 1.75 1.78

EXAMPLE 3 Oxygen Barrier in Laminate with Paper Made of “Wet” SulphitePulp which is Internally Sized Contra Laminate of Similar Paper which isnot Internally Sized

TABLE 4 Results of measurements of OTR of PE-coated HD paper made of“wet” (sulphite) pulp (Candor paper), which is not internally sized withAKD size. 50.9 mN/m. OTR OTR, Stored Polymer Roll pressure kPa ml/m² 24h atm ml/m² 24 h atm CG 8410 250 4.65 5.02 CG 8410 400 4.63 4.53 CA 7230250 6.68 4.77 CA 7230 400 4.96 3.33

Compared with paper which is internally sized, Candor paper gives a lesssatisfactory oxygen barrier. This barrier is nevertheless better thanthat achieved with internally sized paper of paper of higher densitycontaining 60, 90 or 100% of pre-dried sulphate pulp.

1. A method for manufacturing a barrier material with good gas barrierproperties, particularly for use as packaging in the food industry,where the barrier material comprises a laminate of paper coated with apolyolefine, characterised in that the paper is manufactured from wetpulp which has not been permitted to dry up before the paper wasproduced, and before applying the polyolefine to the paper, the paper ismodified in such a manner that its surface energy is changed toapproximate to the same value as the polyolefine's surface energy.
 2. Amethod according to claim 1, characterised in that wet pulp is wood pulpwhere the percentage of water is from 30% by weight or more.
 3. A methodaccording to claim 2, characterised in that the fibrous material iscellulose fibre with a high or medium comminution rate, and that thepaper's surface energy is changed in order to be as similar as possibleto the polyethylene's surface energy.
 4. A method according to claim 3,characterised in that the paper is produced from finely ground wetcellulose pulp with a high content of hemicellulose, such as sulphitepulp or polysulphide pulp.
 5. A method according to any of claims 1-4,characterised in that the paper is treated with steam in the papermachine system before glazing (calendring).
 6. A method according to anyof claims 1-5, characterised in that the paper is internally sized witha neutral hydrophobic agent, such as AKD.
 7. A method according to anyof claims 1-6, characterised in that a roll pressure is employed of atleast 250 kPa, preferably 400 kPa or more when applying the polyolefineto the paper.
 8. A method according to any of claims 1-6, characterisedin that a polyethylene such as LDPE or HDPE or a polypropylene isemployed as a polyolefine.
 9. A method according to claim 8,characterised in that when polyethylene of type HD is used, the paper'ssurface energy is modified before coating so as to be as close aspossible to 38.4 mN/m, and when polyethylene of type LD is used, thepaper's surface energy is modified before coating so as to be as closeas possible to 36.2 mN/m.
 10. A method according to any of claims 1-9,characterised in that several layers are laminated together in order toform a more dimensionally stable packaging product, where at least oneof the layers in the laminate may be composed of a layer of cardboard orpasteboard.
 11. A barrier material for use as packaging, particularlyfor use in connection with foodstuffs, comprising a fibrous layer,preferably of cellulose material coated with a polyolefine on at leastone side, characterised in that it is produced by means of the methodsindicated in claims 1-10.
 12. A barrier material according to claim 11,characterised in that the fibrous material is laminated together withadditional fibrous layers and/or a cardboard material via the outerpolyolefine layer and that the outside of the laminate may be coveredwith additional protective layers, particularly of polyolefine.